The best way to do this is with a program named
Genetica. They offer a free 30 day trial.
You would use the Texture Synthesis feature and the Pattern process. You simply load in a high resolution version of the actual wall and then position the pattern grid to run through the mortar of selected stones. The result is a seamless tile with which you can create any size background at high resolution by stepping and repeating the individual seamless tile. Or, if you prefer, submit the high resolution image to the email address in my signature and I can do it for you for $25.00.